Subject: [Sip] Help: "About multiple early-session sdp body in a request"

From: [email-masked]
Date: Wed, 13 Dec 2006 11:14:00 +0800

Dear Sip Group,

     There encounters some questions when I read RFC 3959 and RFC 3960 
which refer to the Early-Session , that is
 
       1. "Is it available to include multiple early-session sdp body in a 
request?"

    for example:
        Content-Type: multipart/mixed; boundary="boundary1"
        Content-Length: 401

        --boundary1
           Content-Type: application/sdp
           Content-Disposition: early-session

           v=0
           o=Bob 2890844725 2890844725 IN IP4 host1.example.org
           s=
           c=IN IP4 192.0.2.2
           t=0 0
           m=audio 30000 RTP/AVP 0

       --boundary1
          Content-Type: application/sdp
          Content-Disposition: early-session

          v=0
          o=Bob 2890844714 2890844714 IN IP4 host2.example.org
          s=
          c=IN IP4 192.0.2.3
          t=0 0
          m=audio 30002 RTP/AVP 0

       --boundary1--

     2. "How can a UAS decide to handle a early-session sdp ,and not to 
handle other early-session sdps when     UAS receive a request with 
multiple early-session sdps?"

    This question arises from the following scene:
    Before A and B establish communication，
    (1) AS-1 wants to send media to B 
    (2) AS-2 wants to send media to A

    To reach the goal , some principles should obey:
    (1)precondtion defined in RFC 3312
    (2)Early-Session defined in RFC 3959, RFC 3960

    The red circles mark special processing.

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---

From: Paul Kyzivat <[email-masked]>
Date: Fri, 15 Dec 2006 09:46:58 -0500

> which refer to the Early-Session , that is

Note that you may be the first to consider implementing this. I've asked
around and haven't heard of an implementation. So answers are somewhat
speculative.

>        *1. "Is it available to include multiple early-session sdp body 
> in a request?"*

I think this is not entirely clear, but its not clear what it would mean
if you were to attempt it. Also, each offer must be matched to an
answer. If there were multiple early-session offers, then there would
need be multiple early session answers. In that case, there would need
to be some convention to decide which early answer goes with which early
offer.

Getting early-session to work at all is enough of a challenge. Getting
multiple early sessions to work seems like folly.

>      *2. "How can a UAS decide to handle a early-session sdp ,and not to 
> multiple early-session sdps?"*

Well, that is an issue isn't it. It assumes that you don't want to
handle them all. This problem exists with early media in forking
situations without use of early-session, and in fact was one of the
reasons early-session was proposed. If you introduce multiple early
sessions, then in some sense the problem has been reintroduced.

>     (2) AS-2 wants to send media to A

Is this in the context of IMS?

To be able to do this, the AS's must be UAs. For the call flow you show,
both AS-1 and AS-2 must act as B2BUAs. To make sense of all this, and
achieve the sessions you show, I think they must have a lot of
understanding of what is going on in this particular case. In the
abstract I don't see how they could know that, but in a particular case
perhaps that is possible.

In the case you show it appears to me that you might never have to
include two early-session descriptions in the same message. Instead,
wouldn't one be removed, because it is being handled, and another added
to be handled on a different leg? But its hard to say for sure without a
concrete example of exactly what you are trying to do.

IMO you are on a dangerous course in uncharted waters. It would be
better if you found a more mainstream way to do what you want. Since
apparently you have a B2BUA at each step, I think you could probably
achieve what you want in a more straightforward way by carefully
managing the contents of standard offers and answers along the way.

	Paul

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